Currently, the most widely used techniques for geological surveying, particularly in sub-marine situations, are seismic methods. These seismic techniques are capable of revealing the structure of the subterranean strata with some accuracy. However, whereas a seismic survey can reveal the location and shape of a potential reservoir, it can normally not reveal the nature of the reservoir.
It has been appreciated by the present applicants that while the seismic properties of hydrocarbon filled strata and water-filled strata do not differ significantly, their electromagnetic resistivities do differ. Thus, by using an electromagnetic surveying method, these differences can be exploited and the success rate in predicting the nature of a reservoir can be increased significantly.
Consequently, a method and an apparatus embodying these principles together form the basis of the present applicants' EP-A-1256019.
This contemplates a method for searching for a hydrocarbon containing subterranean reservoir which comprises: applying a time varying electromagnetic field to subterranean strata; detecting the electromagnetic wave field response; seeking, in the wave field response, a component representing a refracted wave; and determining the presence and/or nature of any reservoir identified based on the presence or absence of a wave component refracted by hydrocarbon layer.
A refracted wave behaves differently, depending on the nature of the stratum in which it is propagated. In particular, the propagation losses in hydrocarbon stratum are much lower than in a water-bearing stratum while the speed of propagation is much higher. Thus, when an oil-bearing reservoir is present, and an EM field is applied, a strong and rapidly propagated refracted wave can be detected. This may therefore indicate the presence of the reservoir or its nature if its presence is already known.
Electromagnetic surveying techniques in themselves are known. However, they are not widely used in practice. In general, the reservoirs of interest are about 1 km or more below the seabed. In order to carry out electromagnetic surveying as a stand alone technique in these conditions, with any reasonable degree of resolution, short wavelengths are necessary. Unfortunately, such short wavelengths suffer from very high attenuation. Long wavelengths do not provide adequate resolution. For these reasons, seismic techniques are preferred.
However, while longer wavelengths applied by electromagnetic techniques cannot provide sufficient information to provide an accurate indication of the boundaries of the various strata, if the geological structure is already known, they can be used to determine the nature of a particular identified formation, if the possibilities for the nature of that formation have significantly differing electromagnetic characteristics. The resolution is not particularly important and so longer wavelengths which do not suffer from excessive attenuation can be employed.
The resistivity of seawater is about 0.3 ohm-m and that of the overburden beneath the seabed would typically be from 0.3 to 4 ohm-m, for example about 2 ohm-m. However, the resistivity of an oil reservoir is likely to be about 20-300 ohm-m. This large difference can be exploited using the techniques of the present invention.
Typically, the resistivity of a hydrocarbon-bearing formation will be 20 to 300 times greater than water-bearing formation.
Due to the different electromagnetic properties of a gas/oil bearing formation and a water bearing formation, one can expect a reflection and refraction of the transmitted field at the boundary of a gas/oil bearing formation. However, the similarity between the properties of the overburden and a reservoir containing water means that no reflection or refraction is likely to occur.
Thus, EM source such as an electric dipole transmitter antenna on or close to the sea floor induces (EM) fields and currents in the sea water and in the subsurface strata. In the sea water, the EM-fields are strongly attenuated due to the high conductivity in the saline environment, whereas the subsurface strata with less conductivity potentially can act as a guide for the EM-fields (less attenuation). If the frequency is low enough (in the order of 1 Hz), the EM-waves are able to penetrate deep into the subsurface, and deeply buried geological layers having higher electrical resistivity than the overburden (as e.g. a hydrocarbon filled reservoir) will affect the EM-waves. Depending on the angle of incidence and state of polarisation, an EM wave incident upon a high resistive layer may excite a ducted (guided) wave mode in the layer. The ducted mode is propagated laterally along the layer and leaks energy back to the overburden and receivers positioned on the sea floor. The term “refracted” wave in this specification is intended to refer to this wave mode.
Both theory and laboratory experiments show that the ducted mode is excited only for an incident wave with transverse magnetic (TM) polarisation (magnetic field perpendicular to the plane of incidence) and at angles of incidence close to the Brewster angle and the critical angle (the angle of total reflection). For transverse electric (TE) polarisation (electric field perpendicular to the plane of incidence) the ducted mode will not be excited. Since the induced current is proportional to the electric field, the current will be parallel to the layer interfaces for TE polarisation but, for TM polarisation, there is an appreciable current across the layer interfaces.
These phenomena form the basis of the present applicants' WO-A-02/14906 which contemplates a method of determining the nature of a subterranean reservoir which comprises: deploying an electric dipole transmitter antenna with its axis generally horizontal; deploying an electric dipole receiver antenna in an in-line orientation relative to the transmitter; applying an electromagnetic (EM) field to the strata containing the reservoir using the transmitter; detecting the EM wave field response using the receiver and identifying in the response a component representing a refracted wave from the reservoir according to a first mode; deploying an electric dipole receiver antenna in an orthogonal orientation relative to the transmitter; applying an EM field to the strata using the transmitter; detecting the EM wave field response using the receiver and identifying in the response a component representing a refracted wave from the reservoir according to a second mode; and comparing the first mode refractive wave response with the second mode refracted wave response in order to determine the nature of the reservoir.
A horizontal dipole source at the sea floor will generate both TE and TM waves, but the ratio of the amplitudes depends on the direction of propagation relative to the direction of the dipole. In the direction of the dipole, only the TM wave is emitted, whereas in a direction at right angles to the dipoles, only the TE wave is emitted. In between, a mixture of the two modes is emitted, the TM mode dominating for angles with the dipole up to 45° and the TE mode dominating for angles with the dipole from 45° to 90°. Thus, even if the receivers are capable of receiving both modes with equal sensitivity, comparison of the two modes will not be feasible for directions in a certain range around 0° or 90°. This difficulty may be remedied by using, instead of a single dipole source, a multiple dipole source, capable of emitting TE and TM modes of approximately equal amplitudes in all directions simultaneously. The TM mode is influenced by the presence of buried high resistive layers, whereas the TE mode is not. By measuring with the two antenna configurations and exploiting the difference between the two sets of measurements, it is possible to identify deeply buried high resistivity zones, i.e. a hydrocarbon reservoir.
WO-A-02/14906 also contemplates a method of searching for a hydrocarbon-containing subterranean reservoir which comprises: deploying an electric dipole transmitter antenna with its axis generally horizontal; deploying an electric dipole receiver antenna in an in-line orientation relative to the transmitter; applying an EM field to subterranean strata using the transmitter; detecting the EM wave field response using the receiver; seeking in the response a component representing a refracted wave according to a first mode, caused by a high-resistivity zone; deploying an electric dipole receiver antenna in an orthogonal orientation relative to the transmitter; applying an EM field to the strata using the transmitter; detecting the EM wave field response using the receiver; seeking in the response a component representing a refracted wave according to a second mode; and comparing the first mode refractive wave response with the second mode refractive wave response in order to determine the presence and/or nature of any high-resistivity zone.
The first mode may be considered to be a TM mode, and the second mode a TE mode. Thus, measurements are taken with the transmitter and receiver in both in-line and orthogonal orientations, and the two sets of measurements are compared. A characteristic difference in values indicates a highly resistive layer located beneath highly conductive strata. High resistivity indicates the presence of hydrocarbons and so the difference in values is a direct hydrocarbon indicator.